the shells of numerous species of true nautili, and so does the 

 chalk beneath, whilst that, and the oolites lying next below, 

 abound also in ammonite forms, and the still underlying rocks 

 are thickly strewn with other members of the great tribe. 



For the present investigation it is only necessary to dwell 

 principally on two leading forms, the old straight fossil ortho- 

 ceras, and its companion called the cyrtoceras, differing from 

 the former in being slightly curved. 



The chief home of the orthoceras and cyrtoceras is in the 

 Silurian, both are also found in the Devonian. They begin to 

 be supplanted by other genera in the carboniferous limestone, 

 abound in profusion, in the guise of ammonites, in the Jurassic; 

 rapidly decline and become feeble in the tertiaries ; and, save 

 as to the nautilus, are extinct in the present world. 



The shell of the orthoceras appears to have resembled that 

 of the pearly nautilus in that it was divided by shelly par- 

 titions (called septa) into numerous chambers, connected only 

 by a tube called the siphuncle, running through the septa, 

 and terminating in the body of the animal. The latter 

 evidently lived in the last and largest chamber, the other 

 chambers acting as floats, the siphuncle keeping the chambers 

 in a living condition. The shell of the present nautilus is 

 always completely and elegantly curved, whereas that of the 

 orthoceras is always straight. There are other differences, 

 but the argument of the Rede Lecture is founded on this one 

 distinction. It assumes that the straight form became casually 

 curved in some one individual, whence sprung other similarly 

 curved creatures now named cyrtoceras. A multitude of such 

 casual variations, becoming fixed from generation to genera- 

 tion, constituted the cyrtoceras tribe, whilst some other casual 

 adventure or adaptive habit produced further coiling up and 

 corresponding changes, which resulted in the populous races 

 of ammonites and the persistent nautilus. 



We may incidentally remark that both shells, thus claimed 

 as parent and child, have ornaments in the shape of furrows 

 and lines, probably with colour (of wliich some traces have 

 been seen), thus displaying similar regularity and beauty to 

 the features possessed by their modern representatives. It 

 serves still further to connect the present with the remote 

 past, to learn that the shells of these fossil orthoceratidse 

 afford, in some instances, marks of having been broken during 

 life, and repaired again by the animal. The very dawn of life 

 on the earth is chequered by ruin and restoration. The 

 cephalopods were the monarchs of the sea, and, indeed, of 

 creation, for there are no remains of fishes, and we have no 

 trace, in the earliest formations of any land animal. There are 



